Why Solvent is essential for Life?

Water solvent is usually considered and essential basis for any Life in the Universe.
Recently other solvents have been considered (eg Ammonia), but maybe we could come up with an idea of life that doesn't need solvent at all?

As far as I understand solvent is need mainly as medium of TRANSPORTATION of substances and building blocks of life so they could come in contact with one another and react. So if we can imagine environment with exotic way to transport them (mechanical shaking, electromagnetic fields, etc.), solvent is not needed?
I would like open minded response, or firm answer why it is impossible (eg maybe without solvent the building blocks could not react even if brought together mechanically? but why?)

Thanks,
Qshadow.

P.S.
I am talking about some very simple life form of course, maybe on the definition border of life and inanimate matter (like maybe nanobes, but probably not based on our carbon chemistry).

Staff: Mentor

In general any discussion about conditions in which life can appear are highly speculative - we know of only one case, and even there we don't know details, although we have some rough ideas about conditions in which this particular life started.

But the question can be put slightly differently - what are conditions necessary to support chemistry complicated enough. For sure we need a system in which reactions can be fast enough. That means solid phase is rather unlikely, as the transport is limited to very slow diffusion and in general reactions occurs (at least in a reasonable time scale) only on the contact surfaces. So we need something else (which is what you already signaled, I am just putting int here for the sake of completeness).

If not solids, we are left with liquids and gases. Gases are difficult, as most compounds don't become gaseous until they are heated to high temperatures, and they often easily decompose while heated. So in the gaseous phase we are limited to rather simple molecules, and life - at least as we know it - requires rather complicated and variable chemistry.

That leaves us with liquids. Not only presence of solvent allows for a relatively fast transport, but it also makes most of the compounds "transportable" within a reasonably low temperatures.

I can imagine chemistry taking place in a mixture that technically is not a liquid (think supercritical carbon dioxide). But I think even if it is not a liquid, it can be still classified as a solution and a solvent.

Hi Borek, thanks for great answer, you actually answered some questions that I wanted to ask next (like why solid & gas phases are unlikely).
First of all I think the term "complex chemistry" should be defined at least vaguely: maybe certain number of bonds? or number of different elements in one molecule?

I would like to get more into the gaseous phase however as I think here we can maybe come up with a way to have complex chemistry in a gas, first of all it doesn't have to be at high temperature (eg Titan temperature is 94 K and it has very dense atmosphere), also we can always add pressure to increase reaction speed. But again how fast is enough is the big question here, I would like to see some quantatative formula or something that shows that gas can't provide what we need...

Staff: Mentor

Complex chemistry is not precisely defined, but what I mean is that we need many different molecules. "Simple" processes like Kreb's cycle require tens (if not hundreds) of different molecules, and I doubt it is possible to create a life without these numbers of molecules present/used. Just by knowing we need hundreds of different molecules we already put a lower limit at how many bonds or different elements are required (plenty ). Carbon and its compounds are the best known way of achieving required variability.

Note that lowering the temperature and increasing pressure you will convert most of the gases into solids, so we are back at square one. You can't have complex chemistry in a cold gaseous phase, and you can't have complex chemistry in a hot gaseous phase (due to instability of more complicated molecules).

Well definetly Kreb's cycle is out of the question, the idea is to find a very simple cycle that requres few (3-4?) molecules only. Actually I have just found out that using Collision theory and Rate equation I could calculate the upper limit of complexity (in number of molecules used in each cycle) per unit of time in a given gas denisty if I know what to use as minimal sufficient "reaction rate" (I don't).

So the result will be like this: in gas with temperature T and denisty ρ, to get reaction rate of X mol L−1 s−1 the maximum possible molecules in one reaction will be Y.

So if I get that the most complex reaction has 3 molecules, this will be the upper limit of complexity for this gas lifeform chemistry...

However the "rate equations" proved to complex for me to crack in a few minutes I had, especially because I am not a chemistry guy at all and I am missing an understanding of too many basic terms (like order of reaction).

But the bottom line is that I am looking on definite quantitative answer on the maximum possible chemistry complexity in gas enviromnent. I do not agree that we can rule out this enviroment without such calculation.
I am trying to think out of the box here on the possibility of gas as solvent, but also to be as objective and scientific as possible.

Maybe you know that someone already made such calculations and can give me a hint were or what to look?

Staff: Mentor

Simple kinetics doesn't translate into "life". You need much more for that. We don't know the minimum level for the reasons I have outlined in my first post, but it definitely requires more than just a few molecules. Simplest definitions of life require at least ability to growth and reproduce - which means some kind of metabolism. This is not something you can do with 3-4 molecules, this is something definitely more complicated.

I found a proof that gases are good enough to produce the basic building blocks of life even based on our rather complex earth-life chemistry (I think less complex life chemistry unknown to us exists as well).
The famous Miller–Urey experiment produced many building blocks from gas mixture.
So in this case gas worked as solvent right?

Of course from this building blocks to real life is a long way, but we still do not know how it was really done, right?